Making ceramic articles having a high degree of porosity and crushability characteristics

ABSTRACT

A method for increasing the porosity and crushability characteristics thereof embodies the firing of a ceramic compact comprising a reactant fugitive filler material and a ceramic material in a controlled atmosphere.

RIGHTS GRANTED TO THE UNITED STATES OF AMERICA

The Government of the United States of America has rights in thisinvention pursuant to Contract No. F33615-76-C-5110 awarded by theDepartment of the Air Force.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improvements in investment casting and to animproved method for making alumina cores for employment therewith.

2. Description of the Prior Art

The production of directionally solidified (DS) metal eutectic alloysand superalloys for high pressure turbine (HPT) airfoils with intricateinternal passageways for air cooling requires that the core and mold notonly be dimensionally stable and sufficiently strong to contain andshape the casting but also be sufficiently weak to prevent mechanicalrupture (hot cracking) of the casting during solidification and cooling.The DS process requirements of up to 1875° C. for a 16 hr. time periodimposes severe constraints on materials which may serve as mold or corecandidates.

The prior art appears to be mostly limited to the use of silica orsilica-zircon core and mold materials. At temperatures greater than1600° C. the silica based materials fail from the standpoint of bothmechanical integrity and chemical incompatibility with the advancedalloy compositions.

Dimensional control of the silica core is excellent since cristobaliteexhibits very little densification. Microstructural examination revealsthat, in some cases, commercial core compositions employ very largeparticles (>100 μm). The addition of large particles serves to lowerboth shrinkage and mechanical strength.

Paul S. Svec in "Process For Making an Investment Mold For Casting AndSolidification of Superalloys Therein," Ser. No. 590,970, now U.S. Pat.No. 4,024,300, teaches the use of alumina-silica compositions for makingmolds and cores. Charles D. Greskovich and Michael F. X. Gigliotti, Jr.in U.S. Pat. Nos. 3,955,616 and 3,972,367 teach cores and molds ofalumina-silica compositions which have a barrier layer of alumina formedat the mold/metal interface. One possible means for the formation oftheir alumina layer is by a chemical reaction wherein carbon of thesusceptor chemically reduces the material composition of the mold orcore. Charles D. Greskovich in U.S. Pat. No. 4,026,344 also teaches analumina-silica composition wherein the material is of a predeterminedsize so as to favor, and therefore enable, the formation of metastablemullite for molds and cores which exhibit superior sag resistance athigh temperatures.

Aluminum oxide by itself, without a chemical or physical bindermaterial, has been identified as a potential core and mold materialbased on both chemical compatibility and leachability considerations.There is, however, a considerable thermal expansion mismatch between theceramic and the alloy which generates, around the ceramic core, hoop andlongitudinal tensile stresses in the alloy on cooling from the DStemperature. The high elastic modulus and high resistance to deformationat elevated temperatures of dense alumina and its lower coefficient ofthermal expansion than the alloy result in the mechanical rupture or hottearing of the alloy.

A mechanism by which an alumina core body can deform under the straininduced by the cooling alloy must be developed to permit the productionof sound castings. The microstructure of the ceramic core and mold mustbe tailored to permit deformation under isostatic compression at astress low enough to prevent hot tearing or cracking of the alloy. Thesurface of the core and mold must also serve as a barrier to metalpenetration.

The material composition of the core is not only determined by thecasting conditions to be encountered but also by the method ofmanufacturing the core and the method of removal of the core from thecasting.

Should the shape of the core be a simple configuration, one may be ableto make a core by mixing the constituents, pressing the mix into apredetermined shape and sintering the shape for strength for handling.

The production of a core such as required for the intricate internalcooling passages of a high pressure turbine airfoil or bladenecessitates the use of a process such as injection or transfer molding.In injection molding, the molding compound must be capable of injectionin a complex die in a very short time with complete die filling.Furthermore, the molding compound must flow readily without requiringexcessive pressure which could result in die separation and extrusion ofmaterial out through the seams. Excessive pressure must also be avoidedto prevent segregation of the liquid binder and the solids. A sufficientamount of a plasticizing vehicle will accomplish these requirements.However, a primary requirement of an injection molding compound is thatthe volume fraction of solids in the body must be greater than 50% atthe injection temperature. Should the solids loading be less than 50% byvolume, the solids may become a discontinuous phase. Upon removal of theplasticizing material from the core, the lack of particle contact mayresult in deformation of the core specimen. High porosity and,therefore, low density structures in the sintered core specimen arerequired to minimize its compressive strength.

An object of this invention is to provide a new and improved materialcomposition embodying a reactant fugitive filler material for makingcores for casting directionally solidified eutectic and superalloymaterials.

Another object of this invention is to provide a new and improvedmaterial composition for making cores and casting directionallysolidified eutectic and superalloy materials wherein carbon as areactant fugitive filler is employed to increase the porosity andcrushability characteristics of ceramic articles made therefrom.

A further object of this invention is to provide a reactant fugitivefiller which is a primary source of carbon or other suitable reactantmaterial at an elevated temperature to chemically reduce alumina in anew and improved material composition for cores and to produce porositywithin the core.

A still further object of this invention is to provide a new andimproved method for making a core for investment casting with animproved porosity content and improved crushability characteristics.

Other objects of this invention will, in part, be obvious and will, inpart, appear hereinafter.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the teachings of this invention there is provided anew and improved method for making fired ceramic articles suitable foruse in the investment casting of directionally solidified eutectic andsuperalloy materials. The method includes preparing a materialcomposition comprising an organic binder, a reactant fugitive fillermaterial and an alumina flour which is one selected from the groupconsisting of alumina and magnesia doped alumina. Thereafter a portionof the material composition is worked into a preform of a predeterminedshape. The organic binder is then removed from the preform by suitablemeans. The preform is then fired to cause a reaction between the aluminaand the reactant fugitive filler material to provide at least one ormore suboxides of alumina which are caused to be evolved from the firedpreform by a vapor transport action thereby resulting in a ceramicarticle having a predetermined porosity content and enhancedcrushability characteristics.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph showing the morphology of the alumina grainstructure of a fired compact at 500×.

FIG. 2 is a plot of Log of the leaching rate versus theoretical densityof a fired alumina compact.

FIG. 3 is a plot showing the effect of graphite additions on the linearshrinkage of a fired alumina compact.

FIG. 4 is a plot showing the effect of graphite additions on the densityof a fired alumina compact.

FIG. 5 is a plot showing the weight loss due to the reaction betweengraphite and alumina in a fired compact.

FIG. 6 is a plot showing the effect of graphite on the density of afired compact.

DESCRIPTION OF THE INVENTION

With reference to FIG. 1 there is shown the microstructure of firedceramic compact 10 made of alumina. The microstructure shows that theporosity is continuous throughout the compact 10 and that the grainmorphology is characteristic of grains 12 which have undergone vaporphase transport action. The vapor transport action involves theevaporation and/or formation of a gaseous suboxide of a portion ofmaterial of one grain at high surface energy regions of the grain andthe transportation of the material to low surface energy regions of thegrain, where it condenses or is oxidized. By this action the grains 12become coarse and rounded. Additionally, aluminum suboxide gaseousspecies are transported out of the compact 10 whereby the compact 10registers a net weight loss. The vapor transport action results in anetwork of narrow connecting bridges 14 between the alumina particles orgrains 12.

The compact 10 is suitable for use as a core in investment casting ofdirectionally solidified eutectic and superalloy materials. It isdesirable for the cooling passages of the turbine blade to have acomplex configuration. Therefore, it is necessary for the compact orcore to have a complex shape. The preferred method of forming thecompact or core 10 in an unfired state is by injection or transfermolding. The preferred material for the compact or core 10 is alumina ormagnesia doped alumina because casting temperatures are in excess of1600° C. and as high as 1850° C. while directional solidification timesare in excess of 16 hours.

The alumina compact 10 is easily removed from the casting by leaching ina KOH or NaOH solution in an autoclave. The leaching rate, however, isdependent upon the porosity of the compact 10. As shown in FIG. 2, ifone can manufacture a compact 10 with a porosity content of from 60percent to 70 percent by volume, a very significant increase in theleaching rate of the compact 10 can be obtained. Additionally, thecompact 10 would make an acceptable core for making turbine bladeswherein the wall thickness is about 0.060 inch or less since it willhave good crushability characteristics.

In injection molding, the solids content of the material compositionemployed to form a compact to function as a core, and having a complexshape, initially must be in excess of 50 percent by volume to preventthe solids included therein from becoming a discontinuous phase, uponbinder removal and before sintering occurs. Should the solid materialbecome a discontinuous phase the compact may deform or disintegratebecause of insufficient green strength.

To increase porosity in the fired compact a reactant fugitive fillermaterial is desirable. The reactant fugitive filler material provides,along with the alumina material, the total solids content necessary forinjection molding. Upon a subsequent firing at an elevated temperature,the reactant fugitive filler is "burned" off in a suitable manner toincrease the porosity content of the compact 10. A desirable reactantfugitive filler material is one which will also react with the aluminato eliminate or remove a portion thereof from the compact 10 and therebyincrease the porosity content further. Suitable fugitive fillermaterials are those which will provide enough reactant material at theelevated temperature to reduce a portion of the alumina which, in part,is removed from the compact in the gaseous state and which, in part, isdeposited on other alumina grains by vapor phase transport actioncausing a coarsening and a rounding thereof. Preferred reactant bearingmaterials are graphite, aluminum, aluminum carbide, aluminum oxycarbide,boron and boron carbide. Suitable organic materials may also be employedas reactant materials as a carbon source.

The particle size of the alumina is important. It is desirable that thesize of the pores in the compact, particularly at the outside surfaceswhich contact the cast metal, be small enough to prevent any significantmetal penetration. It is desirable that metal penetration of the compactsurface be minimized in order to obtain the best surface possible forthe casting.

The particle size distribution of the alumina has a significant effecton the rheology of the wax-carbonalumina systems. The alumina ormagnesia doped alumina and the carbon bearing material have a particlesize range of less than about 300 microns. The preferred particle sizeis from 1 micron to 50 microns.

ALUMINA POWDERS

Suitable alumina material is obtainable as fused alumina powder from theNorton Company and as aggregate free alumina powder from the MellerCompany. Suitable alumina powders are

(a) Norton-400 Alundum wherein the particle size distribution istypically as follows:

    ______________________________________                                        Particle Size   Weight percentage                                             ______________________________________                                         0-5μ        15%                                                           5μ-10μ    13%                                                           10μ-20μ   64%                                                           20μ-30μ    7%                                                           >30              1%                                                           ______________________________________                                    

(b) Norton-320 Alundum wherein the particle size distribution istypically as follows:

    ______________________________________                                        Particle Size   Weight percentage                                             ______________________________________                                         0-10μ       3%                                                            10μ-20μ   53%                                                           20μ-30μ   36%                                                           30μ-37μ   7%                                                            >37μ         1%                                                            ______________________________________                                    

(c) Norton-38-900 Alundum wherein the particle size distribution istypically as follows:

    ______________________________________                                        Particle Size   Weight percentage                                             ______________________________________                                        0-5             55.5                                                          5μ-10μ    34.0                                                          >10μ         remainder                                                     ______________________________________                                    

(d) Meller 0.3μ aggregate free alumina.

Various possible ceramic mixtures include 80 weight percent Norton-400,balance Meller 0.3μ; 70 weight percent Norton-400, balance Meller 0.3μ;100 weight percent Norton-320; 80 weight percent Norton-320, balanceNorton 38-900, and 100 weight percent Norton 38-900.

Alumina doped with at least 1 mole percent magnesia is also suitable asa ceramic material for making the compact 10. It is believed that theaddition of the divalent alkaline earth cations into the trivalentcation lattice of Al₂ O₃ introduces lattice defects which enhance thekinetics of the dissolution of alumina during autoclave causticleaching.

The magnesia may be present in amounts of from about 1 mole percent upto about 30 mole percent. It has been discovered that as the magnesiacontent decreases, the volume fraction of the magnesia doped aluminaphase increases. The magnesia doped alumina phase encases the spinelphase. The spinel phase therefore provides either an interconnectednetwork defining a plurality of interstices in which the magnesia dopedphase is found or a dispersion of particles within a matrix of magnesiadoped alumina.

Above about 20 mole percent magnesia, the magnesia doped alumina networkbegins to become discontinuous. Dissolution of the alumina network byautoclave KOH or NaOH processing therefore begins to require anexcessive increase in processing time. The decrease in dissolution isattributed to the fact that autoclave leaching must occur byintergranular attack which at a magnesia content of about 25 molepercent is almost an order of magnitude slower than at 20 mole percentcontent.

Two methods of fabricating compacts with magnesia doped alumina may beemployed. In one instance a mechanical mix of alumina powder of thedesired particle size content and the appropriate amount of magnesia isprepared. This mechanical mixture is then added to the melted wax in theprocess to be described later.

In the second instance, the same mechanical mixture is prepared andcalcined at a temperature of 1500° C.±200° C. for about 1 to 4 hours toform a two phase product of spinel and magnesia doped alumina. Thecalcined product is then crushed and ground to a particle size of from 1μm to 40 μm. This mechanical mixture is then added to the melted wax inthe process to be described later.

One or more waxes can be employed to provide adequate deflocculation,stability and flow characteristics. The plasticizing vehicle systempreferably consists of one or more paraffin type waxes which form thebase material. A purified mineral wax ceresin may also be included inthe base material. To 100 parts of the base wax material additions ofoleic acid, which acts as a deflocculent and aluminum stearate, whichacts to increase the viscosity of the base wax, are added. A preferredplasticizing vehicle has the following composition:

    ______________________________________                                        Binder:                                                                              Material            Part By Weight                                     ______________________________________                                        P-21 paraffin (Fisher Scientific)                                                                    331/3                                                  P-22 paraffin (Fisher Scientific)                                                                    331/3                                                  Ceresin (Fisher Scientific)                                                                          331/3                                                  Total                  100 parts                                              ______________________________________                                                      Part By Weight                                                  Additives:                                                                              Material  Range   Preferred                                                                            Typical                                    ______________________________________                                                  oleic acid                                                                              0-12    6-8    8                                                    beeswax,  0-12    3-5    4                                                    white                                                                         aluminum  0-12    3-6    3                                                    stearate                                                            ______________________________________                                    

Despite the addition of deflocculent, large particle size, of the orderof >50 microns, can settle at a rather rapid rate in the wax and canchange the sintering behavior of the remainder of the material mix ofthe molding composition material. The rate of settling of largeparticles is adjusted by varying the viscosity of the liquid medium,wax. To this end aluminum stearate is added to the wax to increaseviscosity by gelling. Increased viscosity also has the additionalbenefits of preventing segregation of the wax and solids when pressureis applied and reducing the dilatancy of the material mixture.

In order to describe the invention more fully, and for no other reason,the reactant fugitive filler material is said to be a carbon bearingmaterial. The amount of carbon bearing material added to the corecomposition mix is dependent upon the porosity desired in the fired coreas well as the average particle size of the alumina material. The carbonmaterial present in the core material mix as graphite has a molar ratioof carbon to alumina of from about 0.3 to 1.25. This molar ratio rangehas been found to provide excellent results. The graphite is retained inthe bisque ceramic during heating until after the alumina begins tosinter and develops strength at the alumina-alumina particle contacts.The graphite can now be removed from the structure, or compact, withoutproducing a discontinuous solid phase that could cause distortion of thecompact.

The expected chemical reactions between alumina and carbon occur attemperatures greater than 1500° C. in a reducing or inert atmosphere.The result of these reactions is the production of volatile suboxides ofalumina. The possible reactions are: ##EQU1## with (2) being the mostprobable reaction to occur.

At temperatures above 1500° C., the vapor pressure of the suboxide issignificant. As the vapor pressure increases, mass transport by anevaporation-condensation type mechanism can occur. If the rate of masstransport through the vapor phase is much greater than mass transport byvolume or grain boundary diffusion, the material is merely rearranged inthe compact and no reduction in the pore volume (i.e. densification) cantake place. In the reducing or inert atmosphere, the suboxide can escapethereby lowering the density of the compact or fired ceramic andproducing the microstructure of the central portion 14 of the compact 10as illustrated in FIGS. 1 and 2.

The effect of carbon additions, in the form of graphite, on the weightloss of the ceramic article when fired in a reducing atmosphere, such ashydrogen, is a function of the heating rate and the atmosphere aboveabout 900° C.

When the heating rate is less than the order of about 100° C. per hourin the temperature range of from about 900° C. to about 1500° C., withoxygen present as an impurity in the controlled atmosphere, the expectedporosity content or the percent decrease in fired density, is notobtained. In fact there is quite a difference noted. Apparently, thecarbon reacts with the gaseous oxygen impurity to form gaseous CO andCO₂, which escape from the compact. Consequently, insufficient carbon isavailable above about 1500° C. to reduce the alumina to a gaseoussuboxide and produce the fired compact of desired porosity content.

Controlled atmospheres for firing the compacts to obtain the desiredchemical reactions in the remaining material may be of a reducing typeor of an inert gas type. Hydrogen may be employed as a reducing gas typeatmosphere. Argon, helium, neon and the like may be utilized foratmospheres of the inert gas type.

As shown in FIG. 3 the effect of carbon additions on the linearshrinkage of the fired ceramic is dependent upon the molar ratio ofcarbon to alumina, the amount of oxygen impurity in the atmosphere andthe heating rate.

As the molar ratio of carbon to alumina is increased the percent linearshrinkage of the compact is decreased. The molar ratio of carbon toalumina may be inadvertently reduced in the compact during the firing ifoxygen impurities in the atmosphere react with a portion of the carbonin the compact to form CO or CO₂. In FIG. 3, the effect of the heatingrate on the oxidation of carbon is shown. When a slow heating rate isemployed, the carbon to alumina ratio is lowered by oxidation of carbonand high shrinkages result. When a fast heating rate is used the carbonto alumina ratio is not greatly affected by oxidation of carbon and lowshrinkages result. If the firing atmosphere were completely free of anyoxygen or water vapor the resulting linear shrinkage would beindependent of the heating rate used and would only be a function of theinitial carbon content. For example, when the carbon to alumina ratio isabout 0.75, the linear shrinkage is only 2% if a fast heating rate ispracticed when the controlled atmosphere includes the presence of oxygenas an impurity therein. In contrast, under the same conditions, with aslow heating rate, a linear shrinkage as high as 13% has been observed.A low shrinkage is desirable in producing the required close dimensionaltolerances. The same effects are noted when undoped or pure aluminaflour is employed in the core composition mix.

The percent linear shrinkage is also dependent on the grain size of thealumina flour employed. A larger grain size material will decrease thepercent linear shrinkage which will occur. Therefore, as statedpreviously, the grain size of the alumina flour employed in making thefired compact 10 is preferably from about 1 micron to about 50 microns.

Referring now to FIG. 4, the molar ratio of carbon to alumina and ofcarbon to magnesia doped alumina, affects the density of the fired core.An increasing molar ratio of up to 0.75 results in decreasing the fireddensity of the ceramic article to about 40 percent of full density froman initial 70 percent of full density when practiced at a fast heatingrate with an oxygen impurity present. However, when a slow heating rateis practiced in the presence of oxygen impurity, the percentage of fulldensity for an article embodying a carbon to magnesia doped alumina offrom 0 to 0.75 remains at approximately 70 percent.

Although the molar ratio of carbon (with the carbon expressed asgraphite) to alumina affects the various physical characteristics of thefired ceramic articles, the rate of heating concomitant with the oxygenpartial pressure also has a pronounced effect on the fired articles.Therefore, an improperly fired ceramic article has less porosity,exhibits poorer crushability characteristics, undergoes highershrinkage, and requires a longer leaching time to remove the ceramicarticle from the casting.

With reference to FIG. 5, the percent weight loss due to the loss ofcarbon and/or alumina is dependent upon the firing temperature. Aboveabout 1550° C., the loss becomes appreciable and is related to molarratio of carbon to alumina. The greater effect is noted with increasingmolar ratios of graphite to alumina.

Referring now to FIG. 6, the effect of the molar ratio of graphite toalumina on the fired density of ceramic articles made from the materialcomposition mix of this invention is shown. For molar ratios of 0.25 to0.75, the fired density increases slightly with increasing temperatureup to about 1500° C. Above 1500° C., the higher molar ratio materialshows a significant decrease in the fired density of the ceramicarticle.

Other suitable starting materials may include rare earth doped aluminawherein the alumina is in excess and the reactant fugitive fillermaterial will reduce the excess alumina present. Such materials includeyttrium aluminate and lanthanum aluminate.

The composition of this invention when prepared for injection moldingmay be prepared in several ways. A preferred method embodies the use ofa Sigma mixer having a steam jacket for heating the contents. When theplasticizer material is comprised of one or more waxes, the wax isplaced in the mixer and heated to a temperature of from 80° C. to 110°C. to melt the wax or waxes. The additive agents of one or moredeflocculents and aluminum stearate are then added, as required, in thedesired quantities. The mixing is continued for about 15 minutes toassure a good mixture of the ingredients. The desired quantity ofreactant fugitive filler material is then added and mixing, at theelevated temperature, is continued until all visible chunks of reactantfugitive filler material are broken up. To this mixture is then addedthe alumina bearing flour or mixture of flours of the desired sizedistribution. Mixing is then continued, in vacuum, at the elevatedtemperature for about 30 minutes or until all constituents areuniversally distributed throughout the mixture. The heat is turned offand coolant water passed through the steam jacket to cool the mixture.Mixing is continued for a period of from 30 to 40 minutes, or until themix is pelletized to a desired size of less than 2 cm.

Employing the composition mix of this invention one is able to injectionmold complex shaped cores at from 200 psi to 10,000 psi and upwards to50,000 psi at temperatures of from 80° C. to 130° C. The shrinkage ofsuch composition mix is on the order of about 1 percent by volume.

The wax is removed from the pressed compact by heating the compact up toa temperature of less than 1100° C. to remove the organic binder. Forexample, the composition is heated to a temperature of several hundreddegrees Celsius until the wax or plasticizer material drains from thecompact. Preferably, the pressed compact is packed in fine alumina orcarbon flour having a finer pore size than that of the pressed compactafter wax removal. This enables the wax to be withdrawn by a capillaryaction induced by the finer pore size packing material. Other suitablepacking materials are activated charcoal, high surface area carbon blackand activated alumina. The wax, as described heretofore, is almostcompletely removed from the pressed compact at about 200° C. Subsequentheat treatment is used to sinter the compact or core material toincrease its mechanical strength for handling purposes. A typicalheating cycle may include a rate of heating at about 25° C. per hourfrom room temperature up to about 400° C. to remove any wax stillpresent in the compact. Thereafter the heating rate practiced is fromabout 50° C. per hour to about 100° C. per hour up to a temperaturerange of from 1100° C. to 1300° C. Between about 900° C. and 1500° C.one may want to heat the compact at a rate of at least 100° C. per hourto prevent excess oxidation of the reactant fugitive filler material.The packed compact is removed from the furnace. Thereafter, the compactis removed from the packing material and the extraneous powder isremoved from the outside surfaces by a suitable technique, such asbrushing. The core is again placed in the furnace for its hightemperature heat treatment. Above about 1300° C., the heating rate isincreased until it is greater than about 200° C. per hour and ispracticed up to an elevated temperature of about 1650° C. or greater,depending upon the end use of the compact, or core. Upon reaching theelevated temperature, isothermal heating is practiced for a sufficienttime for the carbon available to react with the alumina present toproduce the desired level of porosity in the fired compact.

An alternate heating or firing schedule entails a partial removal of thewax from the compact by heating the compact at less than 25° C./hr to atemperature of no greater than 200° C. in packing material. The compactis then removed from the packing powder and placed in the sinteringfurnace. The wax still remaining in the compact gives the compact goodhandling strength. A heating rate of less than 25° C. per hour isemployed up to about 400° C. to remove the remainder of the wax. Inorder to avoid any oxidation of the reactant fugitive filler material,the subsequent heating rate should be as rapid as possible. The compactis thereafter heated at a rate greater than 200° C. per hour up to 1650°C. or higher depending on the end use of the compact. Upon reaching thispredetermined elevated temperature, isothermal heating is practiced fora sufficient time for the reactant fugitive filler material to reactwith the alumina present to produce the desired level of porosity in thecompact.

Any excess carbon in the fired compact is removed by heating the firedcompact in an oxidizing atmosphere at a temperature greater than 900° C.Unbound carbon should be removed from the fired compact to preventpossible "boiling" of the cast metal during the practice ofsolidification of eutectic and superalloy materials.

It is significant to note that when the gases of the controlledatmosphere are completely free of oxygen or water vapor, the heatingrate is of little or no importance.

When the fugitive reactant material is either aluminum or boron, theprobable chemical reactions between alumina and aluminum and boron,include the following:

    Al.sub.2 O.sub.3 +4Al→3Al.sub.2 O                   (3)

    Al.sub.2 O.sub.3 +Al→3AlO                           (4)

    Al.sub.2 O.sub.3 +2B→Al.sub.2 O+2BO                 (5)

    Al.sub.2 O.sub.3 +B→2AlO+BO                         (6)

To illustrate the capability of either boron or aluminum to function asa reactant fugitive filler material, material compositions of aluminaand reactant fugitive filler material were prepared. The molar ratio ofreactant fugitive filler material to alumina was 1:2. The materialcompositions were mechanically mixed and pressed into pellets having adensity of about 60% of theoretical. The pellets were then fired in dryhydrogen, dew point -33° F., and heated to an elevated temperature of1765° C.±20° C. at a rate of 1300° C./hour. The pellets containingaluminum as a reactant fugitive filler material were isothermally heatedat temperature for a period of 1 hour. The pellets containing boron as areactant fugitive material were isothermally heated at temperature for aperiod of 30 minutes. The pellets were removed from the furnace andcooled to room temperature and then examined.

The pellets having aluminum as a reactant fugitive material registered aweight loss of about 20 percent. The pellet density was about 63 percentof theoretical density. The pellets having boron as a reactant fugitivematerial registered a weight loss of about 22 percent. The density ofthe pellets was about 48 percent of theoretical.

The teachings of this invention have been directed towards compactsemployed as cores having a complex shape wherein metal cast about thecore has a wall thickness of the order of 0.060 inch or less. Therefore,"hot cracking" is critical. When the wall thickness of the cast metal isgreater, less porosity is required as the metal has strength to resistthe forces exerted by the core. In such instances porosities less than50 percent by volume can be tolerated. Therefore, compacts for suchcores may be prepared with smaller amounts of the reactant fugitivefiller. Compacts of simple shapes can be made by simple compaction andsubsequent firing following most of the heating sequences describedheretofore for compacts including a wax binder. The compact may comprisean alumina bearing flour of the desired particle size range and areactant fugitive filler to produce the desired porosity content.

We claim as our invention:
 1. A method for increasing the porosity andthe crushability characteristics of cores and molds for investmentcasting including the process steps of(a) preparing a materialcomposition consisting essentially of an organic binder, a reactantfugitive filler material and an alumina flour which is one selected fromthe group consisting of alumina, yttrium aluminate, lanthanum alumina,and magnesia doped alumina, the particle size of the alumina flour andthe reactant fugitive filler material is less than about 300 microns,and the binder consists of less than 50 percent by volume; (b) working aportion of the material composition to produce a preform of apredetermined shape; (c) heating the preform in a controlled atmosphereto remove the organic binder from the preform while retainingsubstantially all of the reactant fugitive filler material therein andto obtain desired green strength, and (d) firing the preform in acontrolled atmosphere to react the alumina and the reactant fugitivefiller material to produce at least one or more suboxides of alumina toproduce a ceramic article having a predetermined continuous porositycontent, grain morphology, and crushability characteristics.
 2. Themethod of claim 1 whereinthe preform of the material composition isheated in a controlled atmosphere at a rate of less than 100° C. perhour to a first temperature range of less than about 1100° C. to removethe organic binder, the preform is heated in a controlled atmosphere ata rate of at least 100° C. per hour through a first predetermined rangeof temperature of from about 900° C. to about 1500° C. to prevent excessoxidation of the reactant fugitive filler material, the preform isheated in the controlled atmosphere at a rate of greater than 200° C.per hour to a second predetermined temperature range of from about 1650°C. to about 1900° C., the preform is isothermally heated within thesecond predetermined temperature range for a sufficient time tochemically react substantially all of the reactant filler material withalumina to form suboxides of alumina, and vapor transporting at leastsome of the suboxides of alumina of the fired ceramic article to obtainthe desired volume of continuous porosity in the fired ceramic article.3. The method of claim 2 whereinthe preform is heated in the secondpredetermined temperature range in a controlled atmosphere of dryhydrogen.
 4. The method of claim 2 whereinthe preform is heated in thesecond predetermined temperature range in a controlled atmosphere of wethydrogen.
 5. The method of claim 4 wherein the preform is heated in thesecond predetermined temperature range in a controlled atmosphere of aninert gas which are selected from the group consisting of argon, heliumand neon.
 6. The method of claim 2 whereinthe reactant fugitive fillermaterial is a carbonaceous material.
 7. The method of claim 2 whereinthereactant fugitive filler material is one selected from the groupconsisting of aluminum, aluminum carbide, aluminum oxycarbide, boron andboron carbide.
 8. The method of either claim 1 or 2 whereinheating thepreform is at a rate of about 25° C. per hour up to about 400° C.,heating the preform at a rate of from about 50° C. per hour to nogreater than about 100° C. per hour from about 400° C. up to atemperature range of from 1100° C. to 1300° C., heating the preform at arate of greater than 200° per hour up to a temperature of about 1650° C.or greater, and isothermally heating the preform for a predeterminedperiod of time at the temperature of 1650° C. or greater.
 9. The methodof claim 8 whereinthe compact is packed in a fine flour up to about1100° C. to 1300° C., the flour is one which is selected from the groupconsisting of alumina, carbon, activated charcoal, high surface areacarbon black and activated alumina, the flour having a pore size whichis smaller than that of the compact after removal of the binder, andremoving the compact from the packing flour and continuing the heating.10. The method of either claim 1 or 2 including the steps ofpacking thecompact in a fine flour which is one selected from the group consistingof alumina, activated alumina, carbon, activated charcoal, and highsurface area carbon black, heating the packed compact at a rate of lessthan 25° C. per hour up to 200° C. to remove at least a portion of thebinder material therefrom, removing the compact from the packing flour,heating the compact at a rate of less than 25° C. per hour up to 400° C.to remove the remainder of the binder material, heating the compact at arate of greater than 200° C. per hour to at least 1650° C., andisothermally heating the compact at the last elevated temperature for apredetermined time period to obtain the desired physical properties. 11.The method of claim 6 whereinthe carbonaceous material is carbon and themolar ratio of carbon to alumina is from 0.3 to 1.25.
 12. The method ofclaim 11 whereinthe preferred particle size of the reactant fugitivefiller material and the alumina material is from 1 micron to 50 microns.